Electronic test system operable in two modes

ABSTRACT

A parametric test system that will provide a forcing current at a predetermined level in one mode or a forcing voltage of a constant potential in another mode, particularly adapted for testing semiconductor devices. The circuit comprises a first amplifier means or voltage compliance amplifier which is an amplifier that produces the voltage compliance necessary for high voltage drive and measurement. The output of this compliance amplifier is controlled by a second amplifier means or control amplifier, both amplifiers being connected between current sources operating at negative and positive voltage levels, and this output is connected to one of two test terminals adapted for connection to the junction of a device under test. The control amplifier is &#39;&#39;&#39;&#39;floating&#39;&#39;&#39;&#39; in that it is referenced to one test terminal so that in the current forcing mode it produces an output that keeps driving the compliance amplifier to a different voltage level which in turn increases current flow at the test terminals. When current through the test junction reaches the predetermined level, set by means of an adjustable resistor, the control amplifier provides the proper output and the system stabilizes, with the first amplifier means maintaining the compliance voltage required. For voltage forcing, an external power source supplies voltage via the control amplifier to one test terminal. As leakage current tries to flow through the junction of the device under test, the potential at the test terminals change and cause an imbalance at the inputs to the control amplifier which changes its output and keeps driving the compliance amplifier to a different voltage level, until it is sufficient to cause leakage current to flow through the device under test. A measuring resistor is provided in the output of the compliance amplifier to facilitate access to test values.

United States Patent McPhail ELECTRONIC TEST SYSTEM OPERABLE IN TWOMODES [72] Inventor: James H. McPhail, Santa Clara,

Calif.

[73] Assignee: American Micro Systems, Inc.,'

Santa Clara, Calif.

[22] Filed: May 10, 1971 [21] Appl. No.: 141,795

Primary Examiner-Gerald Goldberg Attorney-Owen, Wickersham & Erickson 57] ABSTRACT A parametric test system that will provide a forcing currentat a predetermined level in one mode or a forcing voltage of a constantpotential in another 2s-| 90 ig l Nov. 14, 1972 mode, particularlyadapted for testing semiconductor devices. The circuit comprises a firstamplifier means or voltage compliance amplifier which is an amplifierthat produces the voltage compliance necessary for high voltage driveand measurement. The output of this compliance amplifier is controlledby a second amplifier means or control amplifier, both amplifiers beingconnected between current sources operating at negative and positivevoltage levels, and this output is connected toone of two test terminalsadapted for connection to the junction of a device under test. Thecontrol amplifier is floating in that it is referenced to one testterminal so that in the current forcing mode it produces an output thatkeeps driving the compliance amplifier to a different voltage, levelwhich in turn increases current flow at the test terminals. When currentthrough the test junction reaches the predetermined level, set by means;of an adjustable resistor, the control amplifier provides the properoutput and the system stabilizes, with the first amplifier meansmaintaining the compliance voltage required. For voltage forcing, anexternal power source supplies voltage via the control amplifier to onetest terminal. As leakage current tries to flow through the junction ofthe device under test, the potential at the test terminals change andcause an imbalance at the inputs to the control amplifier which changesits output and keeps driving the compliance amplifier to a differentvoltage level, until it is sufficient to cause leakage current to flowthrough the device under test. A measuring resistor is provided in theoutput of the compliance amplifier to facilitate access to test values.

19 Claims, 3 Drawing Figures PATENT ED Nov 14 m2 INVENTOR.

JAMES H, McPHAlL ATTORNEYS PATENTEDmv 14 m2 SHEET 2 [IF 2 mw s NM N

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ELECTRONIC TEST SYSTEM OPERABLE IN TWO MODES This invention relates toan electronic apparatus for forcing electrical current or voltage atpredetermined levels to facilitate the testing of electronic components.

Semiconductor devices must be tested within certain electricalparameters to determine whether they meet design specifications orhavedefects. For example, on multiterminal devices it is necessary to checkeach pin to see if threshold voltage requirements are satisfied, andalso whether any current leakage occurs above acceptable limits withinthe device when I activated. l-leretofore, a problem arose in providinga practical and reliable testapparatus capable of operating in one modeto force current at precisely a predetermined level to enable themeasurement of the voltage required to maintain current -at that level,and of operating in another mode to provide and maintain a forcingvoltage to facilitate the measurement of leakage current. i

Another problem was to provide apparatus capable of providing theaforesaid test functions on semiconductor devices rapidly andaccurately. The later requirement was complicated by the need toeliminate extraneous current values and biasing effects to assureprecise electrical values at test terminals.

Itis therefore one object ofv the present invention to provideanelectronic circuit that solves the aforesaid problems and isparticularly adaptable for use as a parametric tester for multi-junctionsemiconductor devices. 7 j

Another object of the present invention is to provide a circuit thatwill accomplish voltage forcing with current reading and current forcingwith voltage reading for use in testing semi-conductor devices that issimple, compact, has relatively few components and is adaptable formanufacture on a single printed circuit card.

Another object of the present invention is to provide a circuit forparametric testing that will produce outputs with precision andreliability.

More specific objects of the present invention are to provide aparametric testing circuit that will provide a relatively high voltagecompliance for acurrent source; that has full self-limiting control orin other words an automatic compliance limit; and that has .a feedbackcontrol loop which floats within the system in that it is referenced tothe output of the system rather than to ground to allow forcing acurrent with respect to :an actual ground rather than a virtual ground,thereby enabling other equipments to be readily referenced to the testapparatus in systems applications.

Another object of the present invention is to provide .a circuitparticularly adaptable for producing outputs useable in the parametrictesting of semiconductor devices that can be readily adapted to either.N-channel or P-channel operation by changing NPN transistors to PM?transistors and vice versa and reversing the polarity of diodes. Y

The aforesaid objects are accomplished by a circuit comprisinga firstoperational amplifieror voltage compliance amplifier which :is anamplifier that provides the voltage compliance necessary for highvoltage drive and measurement. 'Its output is controlled by a secondoperational amplifier or control amplifier, both of which are connectedbetween a pair of current sources 2 operating at negative and positivevoltage levels. Depending on the operating mode, the circuitprovideseither a predetermined current flow through a pair of output terminals(current forcing mode) or a predetermined voltage potential across theoutput terminals to facilitate the measurement of leakagecurrent throughthe junction (voltage forcing mode). Thejcompliance amplifier has acontrol loop comprising the first current source, a resistor and a zenerdiode thatare virtually connected in parallel by being attached to theinverting and non-inverting inputs of the compliance amplifier. Theresistors is connected to receive a feedback output from the controlamplifier, which. output is produced when the predetermining level ofcurrent flow is reached at the test terminals. The control amplifier isfloating in that it is reference to the voltage level at one testterminal. Its control loop, connected to its inverting and non-invertinginputs, includes a pair of zener diodes connected in series in a leadfrom the second current source and in parallel with the controlamplifier. An adjustable current measuring resistor is virtually inparallel with one of the zener diodes to provide a means for measuringtest voltages and to determine test current values ln the currentforcing mode with a test junction connected to the test terminals of thecircuit, the control amplifier produces an output when current throughthe test junction reaches the predetermined level. This output is fedback to the compliance amplifier to cause it to produce the necessaryvoltage to stabilize the system so that the voltage necessary .tomaintain the current flow can be measured from other terminals of thecircuit. In the voltage forcing mode, an external power source suppliesa voltage to the control amplifier and thusto one test terminal. With adevice under testconnected to this terminal a leakage current will tryto flow from the ground terminal to it and through the measuringresistor. As the leakage current tries to flow, the potential atthe testterminal changes and causes an imbalance at the inputs of the controlamplifier. This provides a change in its output which drives thecompliance amplifier to a potential that causes the voltage potentialacross the measuring resistor to increase until the leakage current willflow. The value of the leakage current may be readily determined byvoltage across the measuring resistor.

Other objects, advantages .and features will become apparent from thefollowing detail description presented with the accompanying drawings,in which:

FIG. .1 is a schematic diagram of a testing device embodying theprinciples of the present invention and arranged for the current forcingmode;

FIG. 2 is a schematic diagram of my testing device arranged in thevoltage forcing mode; and

FIG. 3 is a detailed electrical circuit diagram of a testing deviceaccording to the present invention.

As shown in block diagram form in FIG. 1 a circuit 10 embodying theprinciples of the present invention and arranged in a current forcingmode comprises a first operational amplifier 12 which may. also bereferred to as the voltage compliance amplifier. The output of thisamplifier is supplied through a lead 14 and an adjustable range resistor16, which in this :mode

is a current determining resistor, to anoutput "terminal 18. Anotherterminal 20 is provided which is connected to ground potential. Theseterminals 18 and 20 are adapted for connection to the device under test.

The compliance amplifier 12 is connected by a lead 22 to a negativevoltage source and by a lead 24 to a positive voltage source therebyproviding its operatingpower. This amplifier is referenced to ground bya lead 26. The negative voltage also drives a current source 28connected to a control loop for the amplifier 12. This loop comprises alead branch 30 containing a current determining resistor 32 andconnected to the non-inverting input to the amplifier 12. A lead branch34 containing a zener diode 36 is connected to the inverting lead of theamplifier 12. A feedback lead 38 is connected from the output lead ofthe amplifier 12 to a junction with the branch lead 34 between the zenerdiode and the inverting input.

A second operational amplifier 40 is provided in the circuit which willhereafter be referred to as the control amplifier. Power to thisamplifier is controlled by a pair of zener diodes 42 and 44 which areconnected in series in a lead 46. This latter lead extends from a secondcurrent source 48 that is driven by the plus voltage; the other end ofwhich is connected to the output lead 14 from the compliance amplifier12. A first power lead 50 extends to the control amplifier 40 from ajunction with the lead 46 between the zener 44 and the current source48. A second power lead 52 extends from the amplifier 40 to a junction54 with the lead 46 between the zener 42 and the junction with theamplifier output lead 14. The control amplifier 40 is floating in thatit is referenced by means of a lead 55 to a junction with the lead 46between the two zener diodes 42 and 44. Therefor the amplifier isreferenced to a varying potential in that it varies in accordance withthe potential of the output terminal 18. The inverting input is alsoconnected by a lead 56 to a junction 58 with the lead 46 between thezeners 42 and 44. The non-inverting input to the control amplifier 40 isconnected by a lead 60 to the lead 14 between the resistor 16 and theoutput terminal 18. A first test measuring terminal 62 is connected by alead 64 to the lead 46 between the zeners 42 and 44 and another groundterminal 20a is provided which may be connected to the same ground asthe terminal 20. The output from the control amplifier 40 is suppliedthrough a lead 66 containing a load resistor 68 to a junction 70 withthe lead 30.

The circuit 10, as used in semiconductor parametric test apparatus willprovide a fixed known value of current flow at the output terminals 18and 20. Thus, in a typical test operation these terminals may beconnected to preselected terminals of a device under test (DUT) andcurrent is applied at these terminals with the proper compliancevoltage. For example, to determine the threshold or breakdown voltage (Vof a P- N junction in a metal-oxide-silicon (MOS) semiconductor device,the terminal 18 would be connected to the gate and drain of the deviceunder test, and the terminal 20 would be connected to its source. Thedesired current flow (e.g. l micro-amp) that is to be established andmaintained, is preset by adjustment of the range resistor 16 in anysuitable manner. This preset current level is maintained by providing anegative compliance voltage from the amplifier 12 through the resistor16 to the output terminal 18 as will now be explained. When the circuitis turned on, the current trying to flow through the resistor 16 fromthe ground terminal 20 cannot flow initially across the junction to theterminal 18 because the V or threshold voltage of the test junction ofthe DUI has not been reached. The current also cannot flow through thelead to the non-inverting junction of the amplifier 40 because this hasbeen established as a high impedance input. This condition'causes thevoltage compliance amplifier 12 to go negative. At this point the amountof control current flowing in the resistor 32 in the control loop forthe amplifier 12 is not sufficient to hold the loop stable. This causesan imbalance between the inverting and non-inverting amplifier inputswhich drives the output of amplifier 12 negative in lead 14. Thus, thecompliance amplifier 12 is driven negative until it reaches the V valueof the junction under test plus the voltage drop of the resistor 16which is substantially the same as the voltage drop across the zener 42.For example, assume that the V voltage is 6 volts. When that voltage isreached at the terminal 18, then the preset current (e.g. l microamp)flows through the junction under test. When the circuit 10 wasenergized, operating current for the control amplifier 40 was suppliedby current source 48 through leads 50 and 52. Any current in excess ofthat specified for amplifier 40 passes through lead 46 into zeners 44and 42. The total current originating from the source 48 passes throughthe junction 54, through lead 14 and into the output of amplifier 12. Asstated previously, the amplifier 40 is referenced to lead 46 at ajunction between the zener diodes 42 and 44. Thus, these zeners arecontrolling the plus and minus supply voltages to the amplifier withinthe operating limits. With the amplifier in its operating mode andbefore a device is tested, the terminals 18 and 20 are shorted in astandby condition. This may be accomplished by a suitable switch 72 in aconnector between the terminals. This causes the entire circuit 10 toassume a balanced condition. That is, the voltage difference between theinverting and non-inverting inputs of amplifier 40 is essentially zeroand stable and this causes its output to source a steady current (e. g.1 milliamp) which is required by the resistor 32 and zener 36 for astable condition to exist. This is because zener 36 is virtually acrossthe resistor 32 since the voltage difference between the inverting andnon-inverting inputs of the operational amplifier 12 is essentially zeroand stable. This causes amplifier 12 to maintain at its output junction39 the voltage value of zener 42 only because the terminals 18 and 20are shorted at this point. This same output voltage from the amplifier12 appears across the current determining or range resistor 16, therebyestablishing the desired predetermined current flow at the .testterminals 18 and-20.

Now, the device to be tested is placed in parallel. with the shortbetween the testterminals 18 and 20. When. the short is removed byopening'the switch 72, current wants to flow through the device undertest, thereby causing terminal 18 to go negative with respect to theterminal 20. instantaneously, this causes a voltage imbalance betweenthe non-inverting and'inverting input terminals of the control amplifier40 thereby causing its output to stop sourcing current. This imbalanceremains until the breakdown or V voltage of the device under testis-reached, at which point the forced predetermined current flows'through it and the control amplifier 40 again sources a control current forresistor 32, causing the system toreturn to the stabilized state. Atthis point, the amplifier 12 must supply the same zener voltage atjunction 39 plus the compliance voltage or threshold voltage V requiredto force the desired current into the (DUT). Now, as this current isforced at a constant level thecompliance voltage measurement may be madeconveniently at the terminals 62 and 20a, since it is virtually the samevoltage as across output terminals 18 and 20, and this voltage is theVor threshold voltage of the device under test.

In the voltage forcing mode some alterations are made in the circuit toprovide a modified circuit 10a, as shown in FIG. .2. First, the inputterminals of amplifier 40 are reversed so that its non-inverting inputlead60anow makes a junction 74 with the lead 46 and then terminates at aterminal 76. The inverting input lead 56a is connected directly to thetest terminal 18. In addition, the lead 46 to the output of amplifier 12is opened as by a switch so that it does not connect with the output ofthe compliance amplifier 12. An outside power source such as anoperational amplifier 78, is also supplied which is referenced toground. Its inverting input is connected by a lead 79 to a junction 80in lead 46 between the zeners 42 and 44. The output lead 81 from theamplifier 78 is connected to the lead 52 from the amplifier 40 andprovides a current path for the current source 48. When an externalnegative voltage source (e. g. l 5 volts) is applied to thenon-inverting input lead 82 of amplifier 78, the same voltage exists atterminal 18. This is because the inverting and non-inverting inputs ofamplifier 78 are virtually the same or shorted, and thus its inputvoltage exists at junction 80. Since this junction is tied to junction74 of the non-inverting input of amplifier 40 the same voltage must alsobe at this point, and since the input terminals of the amplifier arevirtually the same or shorted, the voltage at terminal 18 is the same asthe input. to the amplifier 78. Before. the device under test is placedbetween terminals 18 and 20 there is no current trying to flow in theresistor 16. With a steady voltage applied to amplifier 78, thenon-inverting and inverting input terminals to amplifier 40 are zero andstable and it is sourcing a l milliamp output to the current determiningresistor 32 for controlling the compliance amplifier 12..Thenon-inverting.(+) and inverting terminals of amplifier 12 arethereforevirtually zero and stable and the output of amplifier 12 isalso supplying the. same voltageas that supplied at lead 82 to amplifier78 since there is no current flowing through the resistor 16. Now, assoon as a device under test is placed across terminals 18 and 20, acurrent instantaneously attempts to flow through the resistor 16 asdetermined by the leakage of the device under test. This causes theterminal 18 to go more positive with respect to what it was comparedwith the ground terminal20. This causes an imbalance at thenon-inverting and inverting inputs to amplifier 40 and thereby causesits output too reduce the amount of current it is sourcing. The reducedcurrent flow in resistor 32 causes an imbalance in the non-inverting andinverting inputs to the amplifier 12, thereby causing its output to gomore negative in the same manner as in the current forcing mode. Thisincrease in negative voltage tends to allow the leakage current in thedevice under test to flow in the resistor 16, and the imbalance in thecontrol loop of amplifier 12 continues until a voltage is reached acrossresistor 16 that will allow leakage current to. flow. When this occurs,the circuit 10a returns to its balanced stable state, and a measurementof voltage across the terminal 76 and a terminal 84 on the output of theamplifier 12 may be made which is directly related to the leakagecurrent in the device under test.

In FIG. 3, one specific embodiment of electrical circuitry is shown forcarrying out the aforementioned principles of my invention. Thiscircuitry can be made entirely with solid state devices andpackaged on asingle printed circuit card using conventional fabrication techniques.In this embodiment, the current source 28 is comprised of a pair oftransistors Q1 and Q2 that operate to provide a stabilized current tothe amplifier 12. The emitter of transistor Qlis connected to a zenerdiode 90. Its base is connected to a. resistor 92 in parallel with thezener and the emitter of transistor Q2. Thus, the zener voltage of diodeplus the emitter base of transistor Q1 forms a voltage that is placedacross resistor 92 to cause current to flow in the emitter of transistorQ2. This same current flows in the collector of transistor Q2. as anoutput to the amplifier 12 in thelead34.

The current source 48 operates in the same manner as current source 28,with a pair of transistorsQl 1 and Q12 of different polarity since this.current source is connected to a positive voltage. Here, the output oftransistor Q12 goes to the zener diodes 44 and 42 and thus to thecontrol amplifier 40, in the manner previously described. with respectto the diagrams of FIGS. 1 and 2. V

The compliance amplifier 12, as shown includes a pair of transistors Q9and Q10 which have base connections that form its non-inverting andinverting input terminals. The output of the amplifier 12 shown as thejunction 39 is controlled by an output device, namely a transistor Q3whichis connected in a driving arrange ment through its base to theemitter of a transistor Q4 whose base is connected to the collector ofanother transistor Q6.

The output of the current source 28 supplies a load resistor 94, thezener diode 36, a emitter resistor 96 for the transistors Q9 and Q10 andthe current limiting resistor 32 which are parallel connected to receivethe current necessary for proper operation.

The transistor Q6, the drive to the output stage of the amplifier 12, isbase-emitter connected to the input differential pair of transistors Q9and Q10. It is necessary forthe amplifier 12to reach zero voltage with aload on the system. To provide this function a pair of transistors Q8andQ7 are used in combination with a diode 98 to furnish a current into theoutput junction 39of amplifier 12 in order for it to have a current loadat all times.

A transistor Q5 has a current limiting function and Senses the currentin the output line 14 from transistor Q3 through a resistor 100. In thelead 24 from the positive voltage source is a resistor 102 that suppliesbias for the transistors Q6, Q8, Q7, the diode 98 and a resistor 104.Thisresistor 102 also provides a current path from the current source28, through the transistor Q6 and the resistor 94. When the current getslarge enough to cause a diode drop across the resistor 100 thetransistor 05 starts to conduct, and takes current away from transistorQ6. This limits the drive into the output devices Q3 and Q4 butmaintains the current that is presently in the resistor 100. Currentalso flows through the zener 36 and transistor Q8; it also flows throughthe resistor 96 and the differential transistor pair Q9 and Q10; and italso flows as control current through the resistor 32 to the amplifier40. The func tion of the other elements of the circuit may be brieflydescribed as follows. A resistor 106 serves as a bias resistor for thetransistor Q2 and a resistor 108 is a biasing resistor for thetransistor Q12. A resistor 110 and capacitor 112 in series provided afrequency compensating function between the collector and base oftransistor Q6. A resistor 114 is connected to the emitter of transistorQ for current limiting. Another pair of resistors 116 and 118 areprovided in the base connection of transistors Q9 and Q as parasiticsuppressors and yet another resistor 120 between their emitters providesa balance capability. An adjustable resistor 122, provided in serieswith and as part of resistor 32 is used to adjust the control current tothe desired level (e.g. l milliamp). A capacitor 124 is provided inparallel with the zener diode 36 as a bypass and a capacitor 126connected in parallel with the resistor 68 serves as a frequencycompensating element.

As indicated, various leads to output terminals on the circuit of FIG. 3are interconnected by a suitable switching means so that the circuit canbe placed in either a current forcing mode as shown in FIG. 1 or avoltage forcing mode as in FIG. 2. In the drawing shown these switchesare set so that the circuit will provide current forcing and itsoperation in this mode may be described briefly as follows. Assumingthat the current sources 28 and 48 are in operation with a DUT betweenthe terminals 18 and 20, current is supplied to the transistor Q10 ofthe differential pair in amplifier 12 through the junction 70. If thecontrol amplifier 40 is producing an output, it is supplied through thelead 66 to junction 70 through the resistor 68. The amplifier 12functions so that a stable condition is produced whenever there is zerovolts between the base of transistor Q10 and transistor Q9, that is, noimbalance and no tendency for the amplifier to swing in any direction.However, with the DUT between the terminals l8 and 20, the imbalance iscreated at the and inputs of amplifier 40. This causes the controlcurrent in lead 66 to reduce, thereby causing an imbalance in the basesof transistors Q9 and Q10 of amplifier 12. This imbalance causes thevoltage at the junction 70 to go more negative causing the output atjunction 39 to also go negative. This transition to a more negativeoutput is accomplished in the following manner. When the voltage atterminal 70 goes more negative, Q10 conducts less causing O6 to conductless and thereby causing more current to flow into the base of Q4. Thiscauses O3 to conduct more current, drawing terminal 39 close to thenegative supply voltage (V). Now, with terminal 39 going more negative,terminal 58 will follow since it is shorted to terminal 39 through thezener 42. Since there is a virtual connection between the and inputs tothe amplifier 40 the voltage at terminal 18 also goes more negative andthis action continues until the desired current flows in resistor 16.Until this occurs there exists an imbalance at the and inputs of bothamplifiers l2 and 40 causing the voltage at terminals 39 and 18 to gonegative in the manner just described. When the desired current flows inresistor 16, the imbalance in the and inputs to amplifier 40 no longerexists and its output stabilizes amplifier so that it continues toprovide the compliance voltage necessary to maintain or force thedesired current level. In the course of testing devices it often isdesirable to limit the compliance obtainable from the circuit. Forexample, it may be determined that the breakdown voltage occurs in therange of 30 volts for a good device, but it is essential not to exceed50 volts at the device terminals so as to avoid damaging it. Limitingthe compliance voltage to a safe level can be accomplished by applyingthe one milliamp control current from another source directly to theterminal 70 when terminal 18 has reached 50 volts. For example, as shownin FIG. 1, a lead 130 connected to terminal 70 contains zener diode 132connected to ground, and this zener is rated at the limit voltage (e. g.50 volts) plus the voltage rating of zener 42. Thus, in operation, ifthe voltage breakdown is not reached at terminal 70 within the desiredsafe limits, the current determining resistor 32 will draw currentthrough the zener diode 132 rather than from the operational amplifier40. This feature enables the present circuit to provide its testoperations on large numbers of semiconductor devices within safe limitsand thereby greatly increase its versatility.

To those skilled in the art to which this invention relates, manychanges in construction and widely differing embodiments andapplications of the invention will suggest themselves without departingfrom the spirit and scope of the invention. The disclosures and thedescription herein are purely illustrative and are not intended to be inany sense limiting.

I claim:

1. An electronic test system for forcing a constant current through adevice under test comprising:

a pair of test terminals adapted for connection to the junctionterminals of a device under test, one of said terminals being connectedto ground potential;

first amplifier means for providing a compliance voltage having a pairof differential inputs and an out- P an adjustable resistor connectedbetween the output of said first amplifier means and said test terminalthat is ungrounded;

a first current source connected to said inputs of said first amplifiermeans;

second amplifier means also having a pair of differential inputs, one ofwhich is connected to said ungrounded test terminal, and an outputconnected to said first current source and one said input of said firstamplifier means;

a second current source connected to said second amplifier means;

and means connected to the output of said first am plifier means forcontrolling the voltage supplied to said second amplifier means, therebycausing it to vary its output and in turn control the output of saidfirst amplifier means; whereby when said adjustable resistor has beenpreset to a predetermined value, said system will stabilizeautomatically when said first amplifier means arrives at a point whereit supplies the compliance voltage necessary to force the predeterminedcurrent level through the device under test.

2. An electronic test system for providing and maintaining a highlyaccurate source of current and voltage at preselected levels to devicesunder test, said system comprising:

output terminal means adapted for connection to a device under test;

means for supplying a predetermined, selectable output current, saidcurrent supply means including a first control amplifier means havingits input connected to one of said output terminals;

a second amplifier means having its input electrically engaged to theoutput of said control amplifier, said first and second amplifier meansbeing so connected that the system is in equilibrium when the output ofsaid second amplifier means is grounded;

current adjusting means interconnecting the output of said secondamplifier means and the input to said first amplifier means;

means for supplying a predetermined, selectable output voltage; and

switch means connected to said current supply means, said voltage supplymeans, and said output terminal means such that either said outputvoltage or said output current is impressed upon said output terminalmeans and thereby supplied to said device under test.

3. The system of claim 2 further characterized by:

means for limiting the output voltage of said second amplifier means,said limiting means connected to the input to said second amplifiermeans and adapted to maintain the voltage supplied thereto below apredetermined level.

4. The system of claim 3 further characterized by:

connector means interconnecting said output terminal and said controlamplifier means so that said amplifier is referenced to the voltage onsaid output terminal.

5. The system of claim 4 wherein said output means comprises:

a pair of terminals, one of which is connected to said current adjustingmeans and the other is grounded; and

switching means interconnecting said terminals.

6. The system of claim 5 wherein said current control means comprises avariable resistor.

7. The system of claim 6 wherein said limiting means comprises a zenerdiode.

8. An electronic test system for providing and maintaining a highlyaccurate source of current and voltage at preselected levels to devicesunder test, said system comprising:

output terminal means adapted for connection to a device under test;

a current source for supplying a predetermined,

selectable output current;

means for supplying a predetermined, selectable output current;

means for supplying a predetermined, selectable output voltage includinga control amplifier;

a compliance amplifier providing voltage at a predetermined compliancelevel;

feedback means interconnecting the output of said compliance amplifierwith the input of said control amplifier such that the amplifier pair isstable;

means connecting the output of said compliance amplifier to said outputterminal means; and

an outside voltage source connected to the input of said controlamplifier such that the voltage supplied to the input of said outsidevoltage source also appears at said output terminal means.

9. The system of claim 8 further characterized by means for limiting theoutput voltage of said compliance amplifier, said means being connectedto the input of said compliance amplifier and adapted to maintain thevoltage supplied thereto below a predetermined level.

10. The system of claim 9 further characterized by information outputterminals from which leakage currents and breakdown voltages may bedirectly mea sured. I

11. An electronic test system for providing a highly accurate source ofcurrent and voltage at preselected levels to devices under test, saidsystem comprising:

a control amplifier having inverting and non-inverting inputs and anoutput;

a compliance amplifier having inverting and non-inverting inputs and anoutput;

feedback means connecting the output of said compliance amplifier withthe inverting input of said control amplifier;

means interconnecting the output of said control amplifier to thenon-inverting input of said compliance amplifier;

a first switching means for connecting said device under test to eitherthe inverting or non-inverting input of said control amplifier;

current control means interconnecting the output of said complianceamplifier and said device under test;

a second switching means connected in series with said feedback meansfor opening or closing said feedback path; and

means for impressing a variable voltage on the noninverting input ofsaid control amplifier.

12. The system of claim 1 1 further characterized by a negative currentsource connected through a first zener diode to the inverting input ofcompliance amplifier and through a resistor to the non-inverting inputof said compliance amplifier; and

a positive source of current connected through a second zener diode tothe non-inverting input of said control amplifier.

13. The system of claim 12 further characterized by means for limitingthe output voltage of said compliance amplifier to a predeterminedvalue.

14. The system of claim 13 wherein said limiting means comprises a zenerdiode connected between ground and the non-inverting input of saidcompliance amplifier.

15. The system of claim 14 further characterized by output meanscomprising a pair of terminals for electrical connection to said deviceunder test, one of said terminals being grounded, the other beingconnected to said current control means, and switching meansinterconnecting said terminals for shorting them together when saidswitch is closed.

16. The system of claim 15 wherein said current control device comprisesa variable resistor.

17. The system of claim 16 wherein said feedback means is a zener diode.

18. The system of claim 17 further characterized by connector meansinterconnecting the output of said compliance amplifier and theinverting input thereto.

such that'said control amplifier is floating when said switch means isclosed.

1. An electronic test system for forcing a constant current through adevice under test comprising: a pair of test terminals adapted forconnection to the junction terminals of a device under test, one of saidterminals being connected to ground potential; first amplifier means forproviding a compliance voltage having a pair of differential inputs andan output; an adjustable resistor connected between the output of saidfirst amplifier means and said test terminal that is ungrounded; a firstcurrent source connected to said inputs of said first amplifier means;second amplifier means also having a pair of differential inputs, one ofwhich is connected to said ungrounded test terminal, and an outputconnected to said first current source and one said input of said firstamplifier means; a second current source connected to said secondamplifier means; and means connected to the output of said firstamplifier means for controlling the voltage supplied to said secondamplifier means, thereby causing it to vary its output and in turncontrol the output of said first amplifier means; whereby when saidadjustable resistor has been preset to a predetermined value, saidsystem will stabilize automatically when said first amplifier meansarrives at a point where it supplies the compliance voltage necessary toforce the predetermined current level through the device under test. 2.An electronic test system for providing and maintaining a highlyaccurate source of current and voltage at preselected levels to devicesunder test, said system comprising: output terminal means adapted forconnection to a device under test; means for supplying a predetermined,selectable output current, said current supply means including a firstcontrol amplifier means having its input connected to one of said outputterminals; a second amplifier means having its input electricallyengaged to the output of said control amplifier, said first and secondamplifier means being so connected that the system is in equilibriumwhen the output of said second amplifier means is grounded; currentadjusting means interconnecting the output of said second amplifiermeans and the input to said first amplifier means; means for supplying apredetermined, selectable output voltage; and switch means connected tosaid current supply means, said voltage supply means, and said outputterminal means such that either said output voltage or said outputcurrent is impressed upon said output terminal means and therebysupplied to said device under test.
 3. The system of claim 2 furthercharacterized by: means for limiting the output voltage of said secondamplifier means, said limiting means connected to the input to saidsecond amplifier means and adapted to maintain the voltage suppliedthereto below a predetermined level.
 4. The system of claim 3 furthercharacterized by: connector means interconnecting said output terminaland said control amplifier means so that Said amplifier is referenced tothe voltage on said output terminal.
 5. The system of claim 4 whereinsaid output means comprises: a pair of terminals, one of which isconnected to said current adjusting means and the other is grounded; andswitching means interconnecting said terminals.
 6. The system of claim 5wherein said current control means comprises a variable resistor.
 7. Thesystem of claim 6 wherein said limiting means comprises a zener diode.8. An electronic test system for providing and maintaining a highlyaccurate source of current and voltage at preselected levels to devicesunder test, said system comprising: output terminal means adapted forconnection to a device under test; a current source for supplying apredetermined, selectable output current; means for supplying apredetermined, selectable output current; means for supplying apredetermined, selectable output voltage including a control amplifier;a compliance amplifier providing voltage at a predetermined compliancelevel; feedback means interconnecting the output of said complianceamplifier with the input of said control amplifier such that theamplifier pair is stable; means connecting the output of said complianceamplifier to said output terminal means; and an outside voltage sourceconnected to the input of said control amplifier such that the voltagesupplied to the input of said outside voltage source also appears atsaid output terminal means.
 9. The system of claim 8 furthercharacterized by means for limiting the output voltage of saidcompliance amplifier, said means being connected to the input of saidcompliance amplifier and adapted to maintain the voltage suppliedthereto below a predetermined level.
 10. The system of claim 9 furthercharacterized by information output terminals from which leakagecurrents and breakdown voltages may be directly measured.
 11. Anelectronic test system for providing a highly accurate source of currentand voltage at preselected levels to devices under test, said systemcomprising: a control amplifier having inverting and non-invertinginputs and an output; a compliance amplifier having inverting andnon-inverting inputs and an output; feedback means connecting the outputof said compliance amplifier with the inverting input of said controlamplifier; means interconnecting the output of said control amplifier tothe non-inverting input of said compliance amplifier; a first switchingmeans for connecting said device under test to either the inverting ornon-inverting input of said control amplifier; current control meansinterconnecting the output of said compliance amplifier and said deviceunder test; a second switching means connected in series with saidfeedback means for opening or closing said feedback path; and means forimpressing a variable voltage on the non-inverting input of said controlamplifier.
 12. The system of claim 11 further characterized by anegative current source connected through a first zener diode to theinverting input of compliance amplifier and through a resistor to thenon-inverting input of said compliance amplifier; and a positive sourceof current connected through a second zener diode to the non-invertinginput of said control amplifier.
 13. The system of claim 12 furthercharacterized by means for limiting the output voltage of saidcompliance amplifier to a predetermined value.
 14. The system of claim13 wherein said limiting means comprises a zener diode connected betweenground and the non-inverting input of said compliance amplifier.
 15. Thesystem of claim 14 further characterized by output means comprising apair of terminals for electrical connection to said device under test,one of said terminals being grounded, the other being connected to saidcurrent control means, and switching means interconnecting saidterminals for shorting them together when said switch is closed.
 16. ThesYstem of claim 15 wherein said current control device comprises avariable resistor.
 17. The system of claim 16 wherein said feedbackmeans is a zener diode.
 18. The system of claim 17 further characterizedby connector means interconnecting the output of said complianceamplifier and the inverting input thereto.
 19. The system of claim 18further characterized by a fourth switching means interconnecting thenon-grounded output terminal and said control amplifier such that saidcontrol amplifier is floating when said switch means is closed.